In solid state polymerization, or "SSP", a high molecular weight polymer is produced by first preparing a relatively low molecular weight crystallized prepolymer followed by its conversion to a higher molecular weight material in the solid state. A solid phase polymerization process is made possible by the ability of both the starting molecular weight material and the product high polymer to sustain temperatures above the glass transition temperature without fusion of the polymer. The temperature at which the process is conducted must be sufficient to effect chain growth and the attendant increase in M.sub.w.
Solid state polymerization (SSP) of polycarbonates is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073. Typically, SSP involves three steps: a first step of forming a prepolymer, typically by melt polymerization (i.e. transesterification) of a dihydroxyaromatic compound such as bisphenol A with a diaryl carbonate, such as diphenyl carbonate; a second step of crystallizing the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature and its melting temperature. The second or crystallization step as disclosed in these patents is by solvent treatment or heat treatment.
There exists a need for an improved method for crystallizing the polycarbonate prepolymer.
There exists a further need for a prepolymer in a form which may be readily processed to produce polycarbonate having improved properties.
There exists a further need for a process in which the in which the rate of solid state polymerization may be effectively increased.
There further exists a need for a SSP process which may be conducted at shorter processing times.
There further exists a need for a SSP process in which the process may be conducted in fewer stages.